1. Field of the Invention
The present invention relates to stable, thermosetting coating compositions containing polyepoxide resins and a particular type of aromatic urea-based curing agent system therefor.
2. Description of the Prior Art
Polyepoxides, also known as epoxy resins, are a broad class of organic compounds containing the vicinal epoxy or oxirane structure: ##STR1## Polyepoxides are in widespread commercial use as adhesives, castings, surface coatings, electrical insulation and the like. The polyepoxides are not thermosetting when pure, but reaction in the presence of a so-called curing agent can convert them into a resinified, cross-linked, polymeric thermoset plastic.
The primary or secondary amine type of coreactive curing agents, such as diethylene triamine, are extremely fast-acting when used in conjunction with polyepoxide. Once such coreactive curing agents have been introduced into the polyepoxide, the batch must be used immediately, or the resin will harden in production equipment and upon tools. Thus, to obviate waste, the admixture of resin and amine type coreactive curing agents cannot exceed the quantity which is capable of being utilized rapidly.
Attempts have been made to formulate single package, stable polyepoxide-based coating compositions which employ so-called "latent" curing systems. For example, Simms et al; U.S. Pat. No. 3,759,914; issued Sept. 18, 1973 and Pratt; U.S. Pat. No. 3,294,749; issued Dec. 27, 1966, both disclose polyepoxide compositions employing latent curing agents such as particular amine compounds and polycarboxylic acid anhydrides. The curing agents used in the systems of these two patents both require the presence of an additional material such as urea-type compounds which function at elevated temperatures as accelerators for the curing agents in promoting the curing of the polyepoxide resin.
Notwithstanding the existence of coating compositions employing latent curing agents and accelerators therefor, there is a continuing need to develop additional types of stable, single package polyepoxide-containing coating compositions. Such compositions are those which employ fast-acting primary and secondary amines as coreactive curing agents for the polyepoxide resin at elevated curing temperatures but which are substantially free of coreactive amounts of such curing agents when the compositions are maintained at room temperature.